1. Field of the Invention
The present invention relates to an attenuator circuit and, in particular, to a multi-tapped attenuator circuit made up of a semiconductor device and used, in particular, as an electronic volume control for audio devices.
2. Description of the Related Art
FIG. 5 shows a conventional attenuator circuit which is employed as a volume control for audio devices with the use of a digital processing technique. The attenuator circuit comprises series resistor circuits R1 to R8 and parallel resistor circuits R9 and R10. Here, resistors R9 and R10 are expressed as EQU R9&lt;&lt;{R5+R6+R10//(R7+R8),
noting that the symbol "//" means that the resistor R10 is connected in parallel with the resistors R7+R8 , and EQU R10&lt;&lt;(R7+R8).
In this case, an amount of attenuation of the attenuator circuit is determined by a resistance ratio of R1 to R4 and R9 and that of R5 and R6 and R10. In FIG. 5, 1-1, 1-2, . . . show analogue switches of which only one is selectively closed, ATT-IN and ATT-OUT denoting a signal input terminal and signal output terminal, respectively.
Analogue switches 1-1.about.1-9 (S00 to S.infin.) are supplied with one of signal S.infin..about.S00 from a logic circuit and only one of them is turned ON to determine a volume level for audio devices. That is, one resistor circuit is selected by the turned-ON analogue switch 1 to determine an amount of attenuation. A logic circuit of FIG. 8 is controlled by a volume switch external to an audio device, not shown. In this case, a maximum volume level is obtained at an attenuation of zero upon selection of the switch 1-1 (S00). When the switches 1-2, 1-3, . . . are sequentially switched, then a corresponding volume level is decreased with an increasing attenuation level, leading to a zero volume at a maximum attenuation level upon selection of the switch 1-9 (S00).
Now let it be assumed that an output is taken from a junction node of the resistors R5 and R6 in a conventional circuit arrangement. Stated in terms of alternating current, a highpass filter is formed, comprising a capacitor c across a pin and a resultant resistance EQU R6+R10//(R7+R8)
on a reference potential (ground) side, as shown in Figs. A and B, which is situated relative to an output terminal. In this case, the resistor R5 is ignored in the case of a high frequency wave. The frequency characteristic of the filter is determined by the cutoff frequency of f=1/2.pi.CR where R=a resultant resistance. Here a resistance corresponding to R at a junction node of the resistors R4 and R5 is 1.0 k.OMEGA. and, similarly, a resistance corresponding to R at a junction node of the resistors R5 and R6 is 3.16 k.OMEGA., a value which is greater than the aforementioned resistance at the preceding stage seen looking from a ground terminal side.
However, the following problem arises from such a resistance relation. FIG. 7 shows the frequency characteristic of the attenuator circuit when the switches 1-2 .about.1-8 (S10.about.S70) are selected, respectively. At this time, the frequency characteristics of the switches 1-6 (S50) and 1-8 (S70), at a time of switching, in a graph shown in FIG. 7 reveal unduly high levels at a high frequency zone of over 5 kHz. This may be ascribed to the aforementioned resistance relation. The unbalance of the frequency characteristic may pose a problem, such as the volume of a highpass zone becomes louder at the time of a volume control in the audio device in spite of narrowing down a volume range. Thus the circuit of FIG. 5, that is, the attenuator of FIG. 5, is not adequate as an attenuator circuit because the area of the resistors R4.about.R10 is not preferable in a high frequency zone.
Furthermore, if various resistors of a varying value are provided in a common IC, a variation occurs in parts of an associated circuit and hence those resistive components in the IC take a greater occupation area.